This invention relates to the preparation of cadmium mercury telluride and, more particularly, to a method of improving single crystal cadmium mercury telluride by heat treatment.
Cadmium mercury telluride of the general composition represented by the formula Cd.sub.x Hg.sub.1-x Te may be prepared by a number of methods which comprise vapor-phase epitaxy, liquid-phase epitaxy and melt-growth techniques. Such methods as the modified Bridgman and the Harman techniques enable the preparation of homogeneous single crystals of cadmium mercury telluride, hereinafter referred to as CMT.
In spite of careful preparation of single crystals of CMT, defects such as vacancies or dislocations in the crystal lattice and small compositional gradients cannot be entirely eliminated. The prior art discloses a number of methods to reduce these defects and gradients. One of these methods is to subject CMT to a heat treatment or an annealing process. For example, it has been disclosed in U.S. Pat. No. 3,622,405, which issued on Nov. 23, 1971 to J. L. Schmit, that compositional gradients in a body of CMT can be removed by annealing at a temperature which is greater than the solidus temperature and less than the liquidus temperature for the average composition of the body. Further annealing may be done at temperatures below the solidus temperature. In U.S. Pat. No. 3,723,190, which issued on Mar. 27, 1973 to P. W. Kruse et al., it has been disclosed that a CMT alloy is subjected to two heat treatments for controlling the stoichiometry, conductivity type and free carrier concentration. According to this patent, a CMT alloy is heated in a first heat treatment in an evacuated container at a temperature near but below the solidus temperature of the material for a time sufficient to ensure a homogeneous composition and is heated in a second heat treatment in the presence of mercury vapor at a second temperature for a time sufficient to adjust stoichiometry, conductivity type and free carrier concentration.
Although the heat treatments according to the prior art, including the heat treatment in mercury vapor, reduce crystal defects and improve some characteristics of the crystals, certain deficiencies remain. I have observed that single crystals of CMT, which have been subjected to heat treatment in mercury vapor, have variable electrical properties. More particularly, I have observed that considerable variations occur in electron mobility and resistivity in single crystals of CMT.